Previously, coil components, such as a transformer, a choke coil, a reactor or the like, have been widely used in electronic devices.
For example, in such coil components, a typical transformer includes a core (for example, a ferrite sintered product) and a coil bobbin on which a coil is wound. The core is divided into two core members. Each of the core members includes a yoke section and a plurality of magnetic legs (for example, the three of magnetic legs (three legs)). Respective magnetic legs of the two core members are arranged to face each other. In the case of the core member having three magnetic legs (for example, an E-type core), the respective middle magnetic legs (the magnetic leg positioned in the middle of the three magnetic legs, also referred to as ‘middle leg’) of the two core members are inserted into the through-hole of the coil bobbin so that the middle magnetic legs butt (face) each other. In order to adjust inductance, a predetermined gap is typically provided between the butting surfaces (facing surfaces) of the two magnetic legs, which are brought to butt each other.
In the transformer configured as the above, when current flows through the coil on the coil bobbin, the flux of the core changes. Due to the changing flux, the core repeats expansion and shrinkage to a slight amount. This is referred to as magnetostrictive vibration. Such magnetostrictive vibration causes beat note (noise). In order to reduce such beat note, there is known a method of suppressing the magnetostrictive vibration by filling the gap with an adhesive.
FIG. 7 is a cross-sectional view illustrating one of main configurations of a transformer of the background art.
The transformer of the background art shown in FIG. 7 includes a core 2 and a coil bobbin 7 on which a coil 8 is wound. The core 2 is configured by a first core 2a and a second core 2b. 
The first core 2a is a so-called E-type core, which includes a yoke section 3a, side magnetic legs 4a and 5a, which are extended from the yoke section 3a, and a middle magnetic leg 6a. The second core 2b is also an E-type core, which includes a yoke section 3b, side magnetic legs 4b and 5b, which are extended from the yoke section 3b, and a middle magnetic leg 6b. Corresponding magnetic legs of the first core 2a and the second core 2b are arranged to face each other, thereby forming a so-called EE-type core.
The middle magnetic leg 6a of the first core 2a and the middle magnetic leg 6b of the second core 2b are fitted into a through-hole that is formed in the central portion of the coil bobbin 7. The middle magnetic leg 6a of the first core 2a is formed to be shorter than the side magnetic legs 4a and 5a. Therefore, a predetermined gap G (which is from several tens of micrometers to several millimeters, which is also referred to as an air gap) is formed between the butting surfaces 6ap and 6bp of the middle magnetic legs 6a and 6b. The gap G is filled with an adhesive 9. The adhesive 9 fixes the middle magnetic legs 6a and 6b. 
In the transformer of the background art, the adhesive 9, which is disposed in the gap G, may protrude in the lateral direction of the middle magnetic legs 6a and 6b so as to reach the coil bobbin 7 (the protruding portion of the adhesive 9 is indicated by “A”). In this case, the coil bobbin 7 may be fixed to the middle magnetic legs 6a and 6b via the adhesive 9.
The transformer in which the coil bobbin and the middle magnetic legs are fixed via the adhesive has a problem in that the core has cracks or rupture due to stress applied thereto. This occurs due to the low mechanical strength of the core in addition to the following reasons (1) and (2).
(1) Since the coil bobbin and the core have different coefficients of thermal expansion (in general, the coil bobbin is more likely to expand in response to heating), the core is distorted at high temperature.
(2) Since the coil bobbin and the core have different water absorptivities (in general, the coil bobbin has a higher absorptivity and thus is more likely to imbibe), the core is distorted in a high-humidity condition.
As for the reason (1), the temperature of the transformer changes when the transformer is being manufactured as well as when the transformer is operating. That is, the thermal history of the transformer changes overtime when the transformer is being manufactured as well as when the transformer is operating. Therefore, the distortion due to the reason (1) is problematic for both the manufacture and the operation of the transformer. In addition, in thermal expansion/shrinkage test such as heat cycle test, distortion occurs due to the reason (1).
In addition, as for the reason (2), problems generally occur due to temporal changes in moisture after the transformer is shipped as a product.
JP-A-2004-200336 disclose a transformer in which the inner diameter of at least a portion of a coil bobbin that faces a magnetic leg butting portion of a core half structure is formed to be larger than those of the other portions. The inner diameter is increased in order to suppress a protruding adhesive from bonding the core and the coil bobbin to each other.
JP-A-2004-273471 discloses a transformer in which an insulating tape impregnated with varnish is wound on a middle leg. External force on the core, which is generated by drying/curing of varnish, is absorbed by the elasticity of the insulating tape.
JP-A-2010-165857 discloses a technology for suppressing natural vibration of magnetic legs by capping a transformer with a thermal-shrinking tube.